1. Field of the Invention
The present invention relates generally to crystal modulated lasers. More particularly, the invention relates to a resonator configuration which reduces the insertion loss of the electro-optic crystal modulator by compensating for thermal lensing and other effects.
2. Description of Related Art
Conventional crystal modulated lasers, such as waveguide CO.sub.2 lasers, employ an electro-optic crystal positioned directly between the laser gain medium and the outcoupler mirror. The crystal is used to frequency modulate the laser light by applying a periodically changing voltage to the crystal. The periodically changing voltage causes the index of refraction of the crystal to change in synchronism with the modulation voltage. This changes the effective optical length of the laser resonator, causing the frequency of the laser light to be modulated.
In conventional stable resonators, the high intensity laser light quickly heats the crystal causing thermal lensing effects. Thermal lensing results from the fact that the outer periphery of the crystal is in contact or close proximity to a heat removing medium, whereas the center of the crystal is not. This causes uneven heating and a temperature differential is established in the crystal causing the crystal to expand in a nonuniform fashion. The center of the crystal, being hotter, expands more than the outer edges, thereby causing the crystal to take on a lens shape. Thermal lensing is one source of distortion and loss of power in a modulated laser.
Another problem with introducing a crystal modulator in the laser configuration is birefringence. Birefringence is a property of certain crystals characterized by a different index of refraction for different light polarizations. A highly birefringent crystal can rotate light from one polarization to a different polarization, so that the two polarizations interfere with one another and reduce the laser efficiency. Birefringence, thermal lensing and absorption of the laser light by the crystal all contribute to deteriorated laser performance. The introduction of a crystal modulator into the laser cavity thus can reduce the laser power, typically to 1/2 of its previous value. This limits the practical, compact transmitter laser design to about a 10 watt output level using conventional technology.
Due to the foregoing limitations, imposed by the introduction of crystal modulators into the laser cavity, it has been necessary to resort to bulky and inefficient configurations such as the master oscillator power amplifier (MOPA) in order to achieve high power transmitters.